When a liquid metal ion source (abbreviated to LMIS), in which a metal ion is emitted by applying an intense electric field to a molten liquid metal, is used, the ion can be emitted from a minute area, and a minute probe can be focused on a sample while maintaining a high current density. In recent years, LMIS thus characterized by high luminance and minuteness of a probe has become applied for use in the manufacturing industry and in the field of analysis techniques.
The focused ion beam (FIB) is a microfabrication technology provided with LMIS as an ion source. Devices using Ga or In, which is as a low-melting metal, as a material for LMIS have become commercially available, and are widely used as a maskless microfabrication means. FIB is also effective as means for preparing a sample for a transmission electron microscope or the like, and allows the precise and efficient thinning of a specific portion of interest. Further, FIB is very effective as means for observing and analyzing the surface of a sample at a high spatial resolution by detecting a secondary ion generated from the sample by ion irradiation. TOF-SIMS (Time of flight-secondary mass spectrometry) is one of such analysis methods, and can analyze components of the surface of a sample at a spatial resolution in the order of submicrons by measuring the mass of a secondary ion emitted from the sample by FIB irradiation.
As described in U.S. Pat. No. 5,399,865 in detail, for example, LMIS is configured as a hairpin-type or reservoir-type LMIS. As a modification of the reservoir-type LMIS, a capillary needle-type LMIS or the like is known. Further, the same U.S. Patent discloses improved versions of these types.
In the hairpin-type LMIS, a needle-like metal is spot-welded to a curved portion in the center of a hairpin-like metal wire, and the molten metal is stored on the surface of the needle-like metal from near the curved portion by surface tension. The reservoir-type LMIS, in which a reservoir for retaining a liquid metal is placed near an emitter, has a configuration that can store a liquid metal in an amount larger than that of the hairpin-type LMIS. The capillary needle-type LMIS, in which a capillary is formed at the lower end of a reservoir, allows a molten ion material to be easily supplied to an apex of an emitter by utilizing the capillary action between the capillary and the emitter penetrating the capillary. An improved type of LMIS disclosed in U.S. Pat. No. 5,399,865 is characterized in that an emitter portion can be heated by electron bombardment using thermoelectrons from a reservoir heated by current conduction with no molten ion material charged. The emitter is cleaned by this heating using electron bombardment, which can allow a molten ion material to flow in a stable manner.
In order to generate a stable ion beam from LMIS, it is important to supply a molten ion material to the apex of an emitter of LMIS in a stable manner. In all conventional LMISs described above, a molten ion material surely passes through the surface of an emitter when the material is supplied to the apex of the emitter. Therefore, in order to maintain the flow of a molten liquid in a stable manner, the surface of the emitter must be clean and smooth.
In order to generate a stable ion beam from LMIS, at least the apex of an emitter must be maintained in an ultrahigh vacuum atmosphere for avoiding influence by the remaining gas molecules. On the other hand, it is generally known from the viewpoint of the kinetic theory of gas molecules that the gas molecular flux of p/√2πmkT with respect to the partial pressure p is incident on the surface of an emitter. Here, the reference character m denotes a molecular weight of gas molecules of interest, the reference character k denotes Boltzmann constant, and the reference character T denotes a temperature. Therefore, even in a device operated in an ultrahigh vacuum environment at 1×10−10 Torr, when emission has been suspended for a prescribed time or longer, operations such as flushing and aging are imperative for restoring the clean surface of an emitter.
In order to form a minute probe by LMIS, the ion emission area at the apex of an emitter is desirably as small as possible. However, the ion emission area depends on the curvature of a molten ion material covering the apex of the emitter. Therefore, it is difficult to achieve a very minute ion emission area.